Radio frequency power amplifiers typically have significant gain variation over operating frequencies, temperatures and input drive levels. These variations typically have serious implications in a communication system base station where, according to an established standard, a radio frequency signal having a predetermined power level should be transmitted from the transmitting antenna. In response, a power control mechanism is often employed to control the signal power level at the antenna.
The hardware which controls the signal power level often resides in a controller connected to the antenna and power amplifier through coaxial cables. Therefore, to control the signal power level as being transmitted from the antenna, the controller attempts to accurately estimate the amount of signal power loss through cables and connectors, and the power amplifier gain.
Since the gain of a power amplifier is dependent on environmental factors, a feed back method is often employed. In this feed back method, the output power level is sampled by a radio frequency signal coupler. A detector receiving the sampled results produces a signal having a DC level corresponding to the detected power level. The DC signal is compared to a predetermined DC reference signal, and depending on the difference between the produced DC signal and the reference DC signal, the system controller accordingly adjusts the gain of the power amplifier. In this feedback method, however, the produced DC signal representing the actual transmitted power level has been affected by the temperature variation of the power detector, where such a variation can not accurately be introduced in the reference DC signal. As a result, the transmitted power level has inaccurately been estimated.
In a Code Division Multiple Access (CDMA) communication systems, the transmit power level of each carrier must be carefully controlled to utilize the system capacity. In CDMA systems, several carriers at different power levels are combined together before the combined signal is presented to the power amplifier for final amplification and subsequent transmission from the antenna. To control the power level of each carrier, each carrier power level is controlled independently before being combined with other carriers. As a result, the power amplifier gain must be known and maintained at a constant level at all environmental conditions, and power levels of carriers.
Therefore, there is a need to nullify the environmental factors and circuit variations to accurately estimate and maintain the gain of a power amplifier at a constant level in a CDMA communication system.